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1 INTRODUCTION
Azipods, due to a number of obvious advantages
associated with increased propulsion efficiency and
bettermanoeuvrability,areincreasinglybeingusedas
a propulsion and steeringsystemfor many types of
modern vessels from passenger ships and ferries
starting,bytankerstotugsandsmallspecialistunits
serving off‐shore. The azip
o
d propulsion, consisting
of one to three or four propellers which may rotate
about a vertical axis and are powered by electrical
motor, considerably changed methodology of
manoeuvring.
Thisalsoappliestotheshipstoppingmanoeuvre.
This manoeuvre, due to the possibility of using a
numberofdifferentwaystodoso,andva
rious(oft
en
contradictory) opinions about the effectiveness of
individual methods, is one of the primary factors
influencingthesafetyofnavigationofshipsequipped
with the azipod propulsion. This follows directly
fromtheanalysisofthecausesofaccidentsatsea.
Manoeuvresonboardashipmodeleq
uippedwith
azip
ods are a big part of practical exercises carried
out during different ship handling courses at the
Ilawa Ship Handling Research and Training Centre.
TheCentrehasa largemannedmodelrepresentinga
largeLNGcarrierequippedwithtwopullingazipods.
Thetrainingmodelispresentedinphotono1.
Figure 1 The manned model representing a large LNG
carrierusedfortrainingandresearchpurposes
This model is used more and more often to the
training, also the scope of lectures at the request of
participants was changed.They request a clear
reliable opinion on the effectiveness of existing
individual methods ofhandling ships equipped
S
topping of Ships Equipped with Azipods*
J
.Nowicki
TheFoundationforSafetyofNavigationandEnvironmentProtection,Gdańsk,Poland
ShipHandlingResearchandTrainingCentre,Ilawa,Poland
ABSTRACT:Thepapercontainsadescriptionofdifferent possibilities ofstoppingalargeshipeq uippedwith
azipods.Themodeltestswerecarriedouttocomparetheeffectivenessofstoppingtheshipusingthedifferent
methods.TheshipmodelusedinstoppingtestsreproducesalargeLNGcarrierofcapacity~150000m
3
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 8
Number 3
September 2014
DOI:10.12716/1001.08.03.07
*Azipod® is a registered trademark of ABB Oy for ship propulsion units.
374
withazipods.Currently, there is almosttotal lackof
access to results of research and of sea trials.
Nevertheless, according to the prevailing opinion of
MastersandPilots,thecrashstopdistancecanbe50%
ofthesamestoppingdistanceofshipsequippedwith
conventional propeller and the crash stop
distance
dependsstronglyonitsmannerofrealization.
Therefore, in recent months, to acquire the
necessary in training and consulting well‐
documentedknowledge of manoeuvring qualities of
such ships, a series of tests using the LNG carrier
model were performed at the Ilawa Ship Handling
Research and Training Centre. Tests
were aimed at
stopping manoeuvre using the combination of
opportunities provided by the azipod propulsion:
changing the direction of rotation of propellers,
turningofbothazipodsaround,deflectionofazipods
atanangle,etc.
2 STOPPINGMANOEUVREOFSHIPSEQUIPPED
WITHAZIPODS
Inadditiontothethrustofapropelleror
ofpropellers
working astern (case of conventional propulsion),
during the stopping manoeuvre the water reaction
arising on a azipod housing is used. This housing
looks like a rudderbut is less efficient due to
restrictions concerning its shape. A typical
distributionofforcesisschematicallyshowninfig.2.
It can
be seen that the efficiency of the stopping
manoeuvre depends on the optimization of the
azipodpositionandthedirectionofitsthrust.
Figure2.Forcesactingonaazipod
The above mentioned forces depend also on
interaction effects with other azipods and with the
shiphull.Figure3showsschematicallytheinteraction
causes,sothattheoptimalchoiceofazipodangleof
deflectionanddirectionofrotationofitspropelleris
an important factor affecting the efficiency of the
stopping
manoeuvre.
Figure 3. The types of interaction: between azipods and
betweenazipodandtheshiphull
Commonly used vessel stopping modes are
described below. A ship master or pilot can stop a
ship:
by changing the direction of propeller rotation.
Principle is very similar to stopping realized
onboardaconventionalship,
by turning azipods around (180°). Of course this
modeisnotapplicableforships
eq uipped with a
singleazipodbecauseoftheobservedconsiderable
change in the ship’s heading. For twin‐azipod
ships, however, it is quite acceptable and
according to many results of tests this mode of
stopping is more effective than stopping by
changing the direction of propeller rotation..
Undesirableeffectof
thismodeofstoppingisthe
bladeloadingandassociatedstrengths.According
to recent investigations the most dangerous
position of azipod is about 60‐70° from the ship
speedvector.Reducingofthethrustwhileturning
azipodsaround maydiminishthe blade
loading[2].
by indirect manoeuvre. As previously, this
mode
of stopping is not applicable to ships equipped
with single azipod. For ships equipped with two
azipods,itispossibletoturnazipodstoopposite
helm angles while reversing the thrust. The
induced on azipod housings hydrodynamic force
(preciselyitslongitudinalcomponent)canbeused
as a braking force.
Investigations made by
Woodward show, that indirect manoeuvre gives
the shortest stopping time and distance when
comparing with the two previously described
modes of stopping. The optimum azipod helm
angle was 60° [3]. According to report from sea
trials from “Elation” published by Kurimo this
mode of stopping allowed to diminish
the
stopping distance to 2.4 L [3]. Azipod deflection
angleappliedwas35°.
3 THEPROGRAMMEOFMODELTESTS
As already mentioned above, the purpose of the
modeltestswastoobtaindataontheeffectivenessof
different modes of stopping manoeuvre. The tests
programhasbeendevelopedina
mannersuitableto
demonstratethepotentialimpactofinteractioneffect
betweenazipods.
The ship model used for tests has dimensions
showninTable1below:
375
Table1.
_______________________________________________
Length[m]277.5  11.56
Breadth[m]43.2 1.80
Draft[m]12.0 0.50
BlockcoefficientC
B[‐]0.79 0.79
Scale‐1:24
Numberofazipods 2 2
_______________________________________________
The configuration of the stern part of the ship
modelispresentedinfigureno4.Youcanclearlysee
the specific shape of the stern of the ship equipped
with azipod propulsion improving the propulsion
effectivenessand significant skeg area enhancing
thecoursestability.
Figure4.TheLNGcarriermodelequippedwithtwo pulling
azipods used for stopping tests (in this photo the pulling
azipodsareturnedaroundby180degrees)
The model tests were carried out on the lake in
deepwater.Thewindvelocityduringtestswasabout
5 knots and less (after conversion to the real ship
scale).Currentwasnotobserved.
Tests were performed with an initial speed of
about12knots,thatis,forthespeedof
approx.65%of
theservicespeedofatypicallargeLNGcarrier.This
speed does not coincide with recommendations in
standards of manoeuvrability [4], where 90% of the
servicespeedissuggested. Suchachoicewasdictated
howeverbythedemandsofthetraining.
Propellerrevolutionsworkingbackwerethe
same
asintheforwardmotion.Theinstantaneousposition
of azipods and number of revolutions of installed
propellers was also measured. The position of
azipods and number of their revolutions were
changed manually by operators performing
experiment.
The model trajectory, heading and speed were
measuredusingpreciseGPSsystemoperatingin
RTK
mode.
a/. b/.
Figure 5 Comparison of outward (a) and inward (b)
directionofazipodturning
The azipod slew velocity was 7.5 º/s, time for
reversingdirectionofpropellerrevolutionswas 60 s
(bothparametersgivenfortherealship).
Thefollowingstoppingtrialswereperformed:
1 Inertia stopping (propellers windmilling)with the
azipodangleofdeflection90ºoutwards;
2 Stopping with the azipod angle of deflection 90º
outwards.Propellersrevolutionsnotchanged.
3 Stopping with the azipod angle of deflection 90º
inwards.Propellersrevolutionsnotchanged.
4 Stoppingbyturningazipodsaround(180°).When
turningnumberofpropellersrevolutionsreduced,
afterwardspropellerrevolutionsreturnedback..
5 Stopping by changing the propellers direction of
revolution.Asternnumber
ofpropellerrevolutions
thesameasahead.
6 Stopping by indirect manoeuvre: deflection of
azipods 30º outwards, afterwards changing the
propellersdirectionofrevolution.
4 EXPERIMENTALDATA
Resultsofmodeltestsarepresentedintable2below.
The stopping distance is given in non‐dimensional
form as the ratio of track
reach to the shipʹs length
S
d/L (track reach is the total distance travelled along
the ship's path):
Table2.
_______________________________________________
Case DescriptionofthestoppingprocedureSd/L
_______________________________________________
1 Inertiastopping(propellerswindmilling)with  11.2
theazipodsangleofdeflection90ºoutwards
2 Stoppingwiththeazipodsangleofdeflection90º 4.0
outwards.Propellersrevolutionsnotchanged.
3 Stoppingwiththeazipodsangleofdeflection90º 3.8
inwards.Propellersrevolutionsnotchanged.
4 Stoppingbyturningazipods
around(180°).When 2.7
turningazipods,numberofpropellersrevolutions
reduced,afterwardspropellerrevolutionsreturned
back.
5 Stoppingbychangingthepropellersdirectionof 4.4
revolution.Asternnumberofpropeller
revolutionsthesameasahead.
6 Stoppingbyindirectmanoeuvre:deflectionof 3.7
azipodsby30ºoutwards,
withsimultaneous
reversingofpropellers.Asternnumberof
propellersrevolutionsthesameasahead
_______________________________________________
Some recorded trajectories of the model are also
shown.Arepresentedsuccessivelytrajectoriesforthe
followingstoppingmanoeuvres:
Inertiastopping(propellerswindmilling)withthe
azipodsangleofdeflection90ºoutwards
(case described as number 1 in the table no 2,
trajectoryshowninfig.6a)
Stopping by changing the propellers direction of
revolutions (case described as number 5in the
tableno2,trajectoryshowninfig.6b)
Stopping by turning azipods around (case
described as number 4in the table no 2,
trajectoryshowninfig.6c)
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5 CONCLUSION
ExperimentaldatapresentedinTable2showthatthe
azipodpropulsionclearlyprovides a relatively short
stoppingdistance,regardlessofthemodeofcarrying
out the manoeuvre.An average value of stopping
manoeuvrefrom initial velocity of 12 knots is about
3.5L.
Figure 6 Trajectories of stopping manoeuvre for different
modesofitsrealization
Thisisbecause:
Relatively short reversing time (about 60 s)
becauseofpropellerspoweredbyelectricalmotor
comparedwiththeclassicalDieselpropulsionfor
which the reversing time is equal from a few to
severalminutes
Theazipodslewvelocityvariesingeneralbetween
5and7.5°/s,sotha
ttheoppositepositioncanbe
reachedafter24‐36seconds.
The simultaneous use of the hydrodynamic
reaction arising on deflected pod housings
significantlyreducesthestoppingdistance.
Theso‐calledindirectmanoeuvrenotprovedtobe
the best. The non‐dimensional track reach was
about37% greater than stopping by turni
ng
azipodsaround.
The effect of interaction between azipods (see
results of tests using stopping mode no 2 and 3‐
inward and outward azipod turning direction)
increasesslightlytrackreachduringstopping(4L
insteadof3.8L)
Analysisof trajectoriesoftheshipmodelshowed
a significant effect on the model pa
th shape of the
manual method of control of azipod position and
propeller number of revolutions. Even small
unsymmetrical final position of azipods or small
differences during operation of changing their
positionsgive a significant deviation from the
expectedfortwinpodpropulsionstraighttrajectory.
That is need for constant control of the direction of
motionoftheship,usuallyhavingma
nualcontrolof
azipodsystemofpropulsion.
REFERENCES
[1]J.Nowicki Stopping of ships equipped with azipods
AZIPILOTMeeting,Rotterdam2011
[2]V.Ankudinov Summary of Simulation Capabilities
AZIPILOTMeeting,Rome2010
[3]L.Kobyliński, J.Nowicki Ship Handling Course File
SHRTCIlawa2014
[4]IMO: standards for Ship Manoeuvrability, Res. IMO
MSC.137(76).Doc.no.MSC76/23/Add.1‐Annex6,2002
The use of the term Azipod in this article is not
intended to create the erroneous impression that
Azipod is a generic term for marine propulsion
systems. I would like to point out that Azipod® is not a
generic term for podded propulsion units, but a
registered trademark of ABB Oy. The use of Azipod in
the article is based on my practical experience over
many years having shown that the majority of our
customers at the Foundation for Safety of Navigation
and Environment Protection situated in Ilawa, Poland,
in particular masters and pilots of ships, are very
familiar with this term as product name used by the
first and largest manufacturer of podded propulsors,
ABB Oy.”
NOTE FROM AUTHOR